1. Field of the Invention
This invention relates to scanning-beam navigational radio beacon systems.
2. Description of the Prior Art
In a conventional time-reference scanning beam microwave landing system, there is transmitted a single frequency collimated beam which sweeps the service sector of the system, and evaluation of the time of reception of the beam with respect to a separate reference pulse gives angular information. The detected signal in a receiver of the system is related directly to the received power from the scanning beam antenna and, as such, is noncoherent in nature. This does not allow the use of coherent signal processing methods which may be required to achieve necessary accuracy in difficult situations.
The particular type of time-reference, scanning-beam system to which the present invention is applicable is variously described in the technical literature. One useful background reference is a technical paper entitled, Time-Reference Microwave Landing System Multipath Control Techniques, by R. J. Kelley, published in the JOURNAL OF THE INSTITUTE OF NAVIGATION, Washington, D.C. (Volume 23, No. 1, Spring 1976). That reference is particularly useful in understanding the multi-path problem, with which the present invention deals uniquely.
It has been proposed, as described in copending U.S. patent application Ser. No. 662,342, filed Mar. 1, 1976, now U.S. Pat. No. 4,053,892, to supplement transmission of the swept beam by adding a separate, continuous whole of the service sector of a coherence reference signal having a frequency offset from the scanning beam frequency. That expedient provides a phase (coherence) reference which is utilized at a remote receiver of the system to provide a detected signal which is coherent, which, in an elevation system, permits an accurate determination of low angle information. This particular accuracy increase is due to the ability of such a system to discriminate against noncoherent interference, such as reflected multipath signals.